Time delay feature for flame failure protective device



C. M. LATHROP Oct. 14, 1947.

4 TIUE DELAY FEATURE FOR FLAME FAILURE PROTECTIVE DEVICE Filed sept. 4,' 194s Ov A Tum/'na Minimum 72mg May x /lfaxfmsm 72.1: 21./7

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Patented Oct. 14, 1947 TIME DELAY FEATURE FOR. FLAME FAILURE PROTECTIVE DEVICE Carl M. Lathrop, Elizabeth, N. J., assignor to Standard Oil Development Company, a corporation of Delaware Application September 4, 1943, Serial No. 501,330

3 Claims.

`This invention relates to a flame-control safety device and in particular to a device having a variable time safety adjustment.

In many industries it is necessary to maintain flame combustion of fuels as a source of heat. As a necessary adjunct of such combustion, the fuel supply to the burner may be controlled in the event of amefailure in order to prevent explosions and similar hazards. Normally, a flamefailure protective device is employed. Such a device utilizes electric circuits to maintain the fuel valve open when the electrical resistance of the flame is within suitably established values and closed when the resistance of the flame is abnormally low or high or the flame itself is absent. In a particular instrument the distinction between flames of higher than normal conductivity and the total absence of a flame is very difficult to make practically. An industrially acceptable approach in an instrument is the lengthening of the time period between the detection of an unusually high conductivity and the closing of the fuel valve for safety purposes.

The present invention relates to an improved flame protective device. In this device a definite and easily adjustable time delay has been added between the occurrence of a momentary flame condition which-would have unnecessarily closed the fuel valve in prior art equipment and the necessary closing of the fuel valve for safety purposes. erate to sense the presence or supposed absence of flames at the burner equipment. The control of the fuel supply to burners is dependent upon the electrical conductivity of the flame. Though industrially advantageous in many applications, the prior art devices are deficient in failing to distinguish between flames of higher than general conductivity and the total absence of a flame.

An object of the invention ls to furnish a device in which by a simple means a lengthening of the time period can be made and the period of time delay suitably adjusted. Another object of the invention is to furnish a control apparatus for fuel burners in order to avoid explosions and similar hazards which could arise if the burner admits fuel when no flame is present. Another obiect of the invention is to furnish devices having delayed action features which are consistent with safety and reliability of operation.

In Figure l the variation in conductivity with time for a typical flame condition at a burner is presented and also the areas indicated in which the switch in the main circuit controlling the fuel valve of prior art protective devices operates to The present flame control devices opcut off the fuel supply. In Figure 2 a circuit is shown presenting an embodiment of the present invention. Fig. 3 is a diagram showing selective connections for a time delay condenser.

In the past flame protective devices have not been satisfactory. Particularlyin regard to certain momentary fluctuations in the flame resistance which cause the immediate closing of the fuel valve in prior art equipment when there had been no actual flame failure, but merely a change in the electrical resistance of the flame. Thus, the introduction of the time delay periodconsistent with safety and reliability of operation as made convenient in the device of this invention would seem necessary. This is particularly evident with auxiliary burner equipment connected to petroleum cracking units. This auxiliary equipment operates to supply heated gases for bringing themain equipment up to starting tically instantaneous action ofthe prior artede# vices at the lower resistance limit has caused many false or unnecessary closings of the fuel supply valve. Such defects are eliminated by the improved devices of this invention.

It is generally recognized that, according to the quality of the flame, the flame resistance varies between approximately 10 megohms and about 0.4 megohm without the flame being extinguished. However, the combustion of many mixtures, especially the hydrocarbon mixtures which give orange tracers in combustion, give rise to flames of resistance of about 0.4 megohm. A satisfactory time delay device, therefore, should function at the lower resistance after such a time delay period as is consistent with safety and reliability of operation and should have a recovery time sufficiently short to prevent integration over a number of short periods of low flame resistance that would result ultimately in the closing of the fuel valve. The device of the present invention has such advantageous features.

In the embodiment of the invention shown in Figure 2 a customary flame protective device is arranged to actuate switch I0. The switch I0 is maintained closed by the flame protective device during normal operation of the flame in the fuel burner, thereby energizing relay i2 and keeping contact Il closed. With contact i4 of relay l2 closed solenoid Il is energized from terminals il suillciently to hold the fuel valve 2l open and the condenser 22 is charged to a potential V. Terminals Il supply direct current or pulsating direct current power through the flame protective device. The terminals i8 may represent an external source ot either direct or puleating direct current.

Upon the actual failure of the ilame or supposed failure as sensed by the flame protective device, switch i ii is opened and current no longer iiows from the terminals through relay i 2 and resistor 2|. However, upon the opening of the switch I0 the charge then held on the condenser 22 nows through the solenoid of the relay i2 and the resistor 24. The condenser 22 is of sumcient capacity to maintain adequate current through relay I2 as to maintain contact Il closed, thereby preventing the valve 20 from operating until such time as the condenser 22 shall have discharged itself to a point that the poten tial difference then available between the terminals 28 and 30 is insuilicient for relay i2 to maintain contact il closed, at which time the solenoid I6 is deenergized and the valve 2l is closed.

If switch I0 should momentarily open and then close before the potential difference between terminals 28 and 30 becomes too low (as described above) for relay i2 to maintain contact il closed, valve 20 will not close but will continue to remain open.

Determination of the size of the condenser 22, resistor 24 and the resistance of relay I2, as well as the value of potential diiference between terminals 28 and l0 for relay l2 to allow contact Il to open will depend upon the amount of the time delay desired and, therefore, no iixed value is claimed, this being the predeterminable time delay.

In the device of this invention, minor changes in the time delay may -be made by adjusting the tension of the relay spring 32 by means o! the adjusting screw Il. In order to increase the time delay, the spring tension may be reduced, while to decrease the time delay, the spring may be put under greater tension. Major adjustments 4 in the time delay period may be made by changing the wiring of the condenser 22 according to the manner presented in Figure 3, in which various combinations oi' three condensers are shown.

The invention now having been described and illustrated, what is claimed is:

1. In a delayed action flame protective device the combination which comprises a name responsive device sensitive to the varying conductivity oi' a llame, an electric circuit, an electric switch in such circuit operated by said llame responsive device, an electromagnetic relay and a resistor connected in sexies in said circuit, condenser means oi' predetermined capacity for storing electric energy connected in parallel with said relay and resistor, a second electric circuit, a fuel valve operated by said second circuit, and an electric switch in said second circuit operated by said electromagnetic relay.

2. A delayed action name protective device according to claim 1 in which said electromagnetic relay comprises an adjustable spring for biasing the last-named switch to open position.

3. A delayed action iiame protective device according to claim 1 in which said condenser means comprises combinations of individual condensers of small capacity, and selective connections between said condensers for prolonging or reducing the period of the delayed action.

CARL M. LATHROP.

REFERENCES CITED The following references are of record in the Ille of this patent:

UNITED STATES PATENTS Number Name Date 1,330,507 i Willenbruch Feb. 10, 1920 1,970,455 Humphries Aug. 14, 1934 2,156,630 Knowlton May 2, 1939 1,182,637 Marbury Dec. 5, 1939 2,205,676 Stansburg et al June 25, 1940 2,210,669 Johnson s Aug. 6, 1940 2,214,020 Harwood Sept. 10, 1940 2,112,736 Cockrell Mar. 29, 1938 2,127,977 Lamb Aug. 23, 1938 2,136,256 Sweet Nov. 8, 1938 

